A magnetoresistance (MR) sensor is used in a read/write head to read magnetic fields on a recording medium of a magnetic storage device. An example is the read/write head of a computer hard disk drive or a magnetic recording tape drive. The read/write head is positioned closely adjacent to the recording medium in the case of the computer hard disk drive, separated from the recording medium by an air bearing, or even touching the recording medium. A data bit is written onto an area of the recording medium by locally changing its magnetic state using the writing portion of the read/write head. That magnetic state is later sensed by the MR sensor, which is the reading portion of the read/write head, to read the data bit.
Two known types of MR sensors are a giant magnetoresistance (GMR) sensor and a tunnel magnetoresistance (TMR) sensor. The general technical basis, construction, and operation of the GMR sensor are described, for example, in U.S. Pat. No. 5,436,778. The general technical basis, construction, and operation of the TMR sensor are described, for example, in U.S. Pat. No. 5,729,410. The disclosures of both patents are incorporated by reference in their entireties. These patents also describe the read/write heads and the magnetic storage systems.
The structure of the MR sensors, such as the GMR sensor or the TMR sensor, includes two thin-film stacks separated by an intermediate nonmagnetic film. The intermediate nonmagnetic film is typically a copper film or an aluminum oxide film, serving as a spacer layer for the GMR or the TMR sensors, respectively. In one form, the lower thin-film stack includes a transverse (perpendicular to an air bearing surface of the sensor) magnetic biasing structure, and the upper thin-film stack includes a sensing stack with a free layer that responds to an external magnetic field. A longitudinal (parallel to the air bearing surface) magnetic hard biasing structure is present, either as part of the upper thin-film stack or positioned laterally from the thin-film stacks. These stacks may be inverted, as well.
An overlayer or cap layer is deposited over the thin-film stacks. The performance of the MR sensor depends upon the nature of the overlayer. Although a number of different overlayers have been utilized and are operable, they do not provide an optimum combination of performance and stability. There is accordingly a need for an improved overlayer structure. The present invention fulfills this need, and further provides related advantages.